1. Field of the invention
The present invention relates to a local area network interconnection system comprising at least two local area networks interconnected via a wide area network, certain interconnection nodes being formed by at least two redundant pieces of interconnection equipment, each piece of interconnection equipment having a specific physical address.
The invention likewise relates to a piece of network interconnection equipment intended to be used in such a system.
The invention has highly significant applications in the field of company local area network interconnections. For example, such an interconnection system is very often used for interconnecting agencies of a business with the remote servers of its headquarters via pieces of interconnection equipment of the bridge/router type. In this case the end local area networks and the wide area network (Ethernet and X25 in most cases) provide great reliability, and the weakest link in the chain of the system is formed by the pieces of interconnection equipment particularly on the side of the servers.
2. Discussion of the Related Art
To improve their reliability, it is known that a plurality of redundant IP routers are used for the critical pieces of interconnection equipment, while the system thus formed is managed on the basis of exchanges of specific ICMP messages between said IP routers and the stations of the local area networks connected to these routers. This ICMP message exchange mechanism is described in the document "Request For Comments 1256", published by IETF, September 1991 and entitled "ICMP Router Discovery Messages".
More particularly, these specific ICMP messages which are encapsulated in IP frames, make it possible for the stations of the various local area networks to learn the addresses of the IP interfaces of the routers which are located in the Internet subnetwork to which they belong. Such messages are periodically transmitted by the routers to the stations. In each message can be indicated various addresses with an degree of preference; for transmitting their frames the stations will utilize the address that has the most preference at a specific instant. Each message additionally comprises a field which indicates the validity limit of the addresses transmitted in the message, so that the stations discard addresses of the routers that have a breakdown or are no longer accessible.
Thus, a plurality of IP routers can be connected redundantly. They periodically transmit each specific ICMP messages which indicate their address and the degree of preference that has been assigned to them. For transmitting their frames, the stations of the local area network use the router whose address has the most preference. When this router becomes inaccessible (in the case of a breakdown of the piece of interconnection equipment or of the link, for example), the stations no longer receive the ICMP messages from the router and, consequently, when the validity of the last received message has expired, they discard the existence of the router and direct their frames to the remaining router that has more preference.
This method, however, has the great drawback of being applicable only to pieces of interconnection equipment of the IP router type toward stations which emulate the IRDP protocol (ICMP Router Discovery Protocol).